Abstract: Egg yolk liquid is thermally sensitive and begins to denature at around 64 ℃, forming a gel. For this reason, heat treatment during processing can deteriorate the emulsifying properties of egg yolk liquid, limiting its application in the food industry. In this experiment, the effects of different heat treatments (at 60, 65 or 70 ℃ for 4 min) on emulsifying properties and protein structure of enzymatically modified egg yolk liquid were studied. The results showed that the emulsifying activity index (EAI) of egg yolk liquid decreased with the increase of heat treatment temperature, and the average particle size of the emulsion system formed increased. Under the same heat treatment condition, the EAI and emulsion stability index (ESI) of the modified egg yolk liquid were significantly higher than those of its unmodified counterpart (P < 0.05), and the particle size of the emulsion system formed with the modified egg yolk liquid decreased significantly, and the distribution of particle sizes was closer to the normal distribution. Raman spectral analysis showed that the secondary structure of proteins in the modified yolk liquid changed after heat treatment. With increasing heat treatment temperature up to 65 ℃, the relative content of α-helix structure increased significantly and the relative content of random coil structure decreased markedly (P < 0.05). Meanwhile, tryptophan and tyrosine residues and aliphatic hydrophobic groups were more exposed to the outside of protein molecules than the unmodified one. Partial least squares (PLS) was used to establish a model for describing the emulsifying properties of egg yolk liquid based on the Raman spectral data. It was found that changes in the conformation and secondary structure of proteins (especially low density lipoprotein) in egg yolk liquid had the greatest influence on the emulsifying properties of egg yolk liquid.

Key words: egg yolk liquid, enzymatic modification, emulsifying properties, Raman spectroscopy, heat treatment